1. Technical Field
The invention relates to data communications and more particularly to data communications over a wide area network implemented on a telecommunications system imposing tariffs. Still more particularly the invention relates to a method of minimizing connection costs between stations over the telecommunications system where the stations exhibit bursty or intermittent data transmission demands.
2. Description of the Related Art
Data communication over a public data network is facilitated by careful definition of the interface between terminal equipment, such as a user's personal computer, and the network. Such a definition is part of a protocol, a set of semantic and syntactic rules that determines the behavior of functional units in establishing communication between two terminals. A number of such protocols have been written which are usable with wide area networks. Among these protocols are the Open Systems Interconnection architecture (OSI), circuit switched X.25 and the System Network Architecture (SNA). While the present patent has general applicability to wide area networks implemented on public data networks, it is particularly applicable to a network service called the Integrated Services Digital Network (ISDN), which can be implemented with any of a number of protocols.
ISDN is a digital end-to-end telecommunication service that supports multiple services including, but not limited to, voice, interactive video and data. Among the data networks on which ISDN may be implemented is the telephone system in place in Canada and the United States as of the 1990's. This telephone system utilizes digital transmission channels between switching exchanges and analogue links between subscribers and those switching exchanges. Analogue to digital and digital to analogue conversion is provided at the switching exchanges. ISDN provides modification of the physical link between a terminal and the telephone central switching office so that it can support digital communication. With such a modification, the use of modems for linking computers over telephone lines can be eliminated. ISDN provides greatly increased data transmission rates, in part by cutting out the multiple conversions between digital and analogue representations of a signal occurring at the central switching office and in the modems collocated with the terminals. Communication is digital from terminal to terminal.
An ISDN interface uses the physical link to the telephone system central switching office to provide a subscriber a plurality of logical channels to the central switching office and through it to remote terminals. The channels are time division multiplexed logical channels. One logical channel, called the "D-channel", is between a terminal and control circuitry of the central switching office. The D-channel is used primarily for passing control information between the terminal and the local ISDN switch in the network. A plurality of logical "B-channels" are also available. These are used for end-to-end connections for data transmission between terminals. End-to-end connection is controlled by D-channel activity. Each B-channel carries data at rates up to 64 Kbits per second. With the ISDN basic rate two B-channels and one D-channel are available. With the ISDN primary rate, 23 or more B-channels are available.
In network architecture, a group of services that is complete from a conceptual point of view and that extends across all systems that conform to the network architecture is called a layer. As implied by the term layer, such groups are arranged hierarchically. In ISDN, stacks of layers are provided for each channel. For the D-channel there is the control protocol stack and for each B-channel in use there is a user protocol stack. The layers present in a user protocol stack depend upon the protocol in use. In other words, a terminal may belong to more than one network simultaneously. A typical hierarchy for a user protocol stack includes an application layer (layer 7), a presentation layer (layer 6), a session layer (layer 5), a transport layer (layer 4), a network layer (layer 3), a data link layer (layer 2) and a physical layer (layer 1 ). Layer 1, at the bottom of the hierarchy, changes little from protocol to protocol (or between control and user protocol stacks). Layer 1 provides the physical means to implement a connection between a terminal and the telephone central switching office. Layer 1 functions are defined in CCITT (the International Consultative Committee on Telephone and Telegraph) Recommendations I.430 and I.431.
Layer 2 services have typically included insertion and deletion of transmission and receive bits from frames, the interpretation of address and control fields and the generation and interpretation of frame check sequences (relating to error detection and correction). Decisions on whether to establish or disestablish a link to another terminal on a B-channel have occurred at higher layers, typically the application layer.
In one sense, communications over the B-channels are not so much between terminals as between users of the terminals or between applications executing on the terminals. The activities of the lower layers are driven by events at the application layer at the top of each of the user protocol stacks. Communications are required when remote resources are needed to answer a user inquiry or when a message from a user is to go to another terminal. Many applications produce bursty communications traffic, especially interactive applications involving transmission of images. Bursty communications mean that frames are bunched in time. At times a B-channel is busy while at other times the channel is not carrying traffic. However, if the B-channel remains dedicated it is not available for use by another application. Of potentially greater consequence in terms of expense, tariffs imposed by the owner of the public data network accumulate for each B-channel in use.
Tariffs are fees for use for a specific unit of service provided by the telecommunications facility. In North America, fees are imposed for a switched-circuit connection per unit time of connection with a charge for an initial minimum unit. In other words, for an ISDN subscriber, variable fees are imposed for each B-channel when in connection whether carrying traffic or not. No fee is imposed for use of the D-channel. There exists potential for reducing telephone charges to ISDN subscribers if B-channel connection time can be limited to those periods where needed to meet data traffic. However, the existing protocols do not provide such a tool. Further, such a tool would preferably be compatible with existing protocols.
Selective connection to reduce connection costs has been used with unbalanced protocols, i.e. wide area networks where pairs of terminals in communication with one another are in a master-slave relationship. An example of such a system is the X.21 switched network, for which International Business Machines Corporation developed a Short Hold Mode (SHM) protocol. SHM is based upon short connection durations and uses frame formats that combine Systems Network Architecture (SNA) information and connect/reconnect information. SHM procedures are based upon and intertwined with Synchronous Data Link Control (SDLC) procedures. SDLC is a layer 2 protocol that requires communicating devices to be designated primary or secondary. Primary devices than poll secondary devices as a prerequisite to secondary devices transmitting data. In SHM temporary disconnection occurs after a predetermined number of non-productive polls have occurred. It cannot be used in any strategic balanced layer 2 protocol such as CCITT Q.922/IBM IDLC, X.25 HDLC, or IEEE 802.2, which do not normally involve polling. Nor can SHM be used with other high level protocols such as Open Systems Interconnection (OSI).
Before summarizing the invention, three terms used here should be defined. One term is "frame", which is a data structure that consists of fields, determined by the particular protocol, for the transmission of application, control, or user data. A frame is a standardized way of organizing data, which includes areas for insertion of information. A circuit-switched connection is a connection established and maintained on demand between two data stations. A balanced protocol at layer 2 is one where either node in a session may initiate data transmission without any polling.